In the context of audio conferences, several audio stream interchange configurations can be envisaged, notably:    distributed-mode multipoint links in which each terminal is responsible for receiving, processing the audio streams that it receives from the terminals of the other participants and sending its audio stream to the latter.    centralized-mode multipoint links in which a sending entity, generally called “conference bridge”, manages, within the network, the audio streams addressed to the receiving terminals of the participants. The use of a bridge in an audio conference presents numerous advantages:    reduced signalling message traffic, for example SIP in VoIP mode,    easier management of the conference because of the possibility of inviting or removing participants, broadcasting information, in particular to the participants party to the call, and easily controlling and administering the conference,    for the operators, simplified billing for the services since it is possible to know at all times who is present or absent in the conference.
The invention applies to bridge-centralized multipoint links.
The principle of the bridges is to enable the conference to take place by using a central point either to combine the audio streams according to the mixing principle, or to duplicate and transmit the audio streams according to the replication principle. Two types of bridge are therefore distinguished: bridges operating in mixer mode, or mixer bridges, and bridges operating in replicating mode, or replicating bridges.
According to the mixer bridge operating principle, each transmitting terminal sends an audio stream to the bridge, whether it be of mono, stereo, multichannel, or other type. For each receiving terminal, the bridge mixes all the streams received from the other participants, omitting the contribution of the receiving terminal concerned if it has transmitted a stream. Thus, of N terminals in a conference, each sends its stream to the bridge and receives from the latter a mixed stream from the N−1 others.
The replicating bridge operating principle consists, for each transmitting terminal, in sending an audio stream to the bridge, whether it be of mono, stereo, multichannel, or other type. At the bridge, the stream from each transmitting terminal is transmitted to all the other participants. Thus, each terminal receives from the bridge N−1 streams corresponding to the N−1 other terminals.
It can then be seen that these two types of bridge present very different constraints, according to the number N (it is assumed that N>1) of participating terminals, in terms of bit-rate and CPU computation unit costs for the bridges and the terminals:    The bit-rates outgoing from the bridges are respectively proportional to N for the mixer bridge and to N(N−1) for the replicating bridge. The bit rates incoming at the bridges are proportional to N in both cases.    The bit-rates incoming from the terminals are respectively proportional to 1 for the mixer bridge configuration and to N−1 for the replicating bridge configuration. The bit-rates outgoing at the terminals are proportional to 1 in both cases.    The CPU costs for the mixer bridge are proportional to 2N, that is, N encodings and N decodings. By contrast, they are zero for the replicating bridge.    The CPU costs for the terminals are respectively proportional to 2 for the mixer bridge configuration, or 1 encoding and 1 decoding, and to 1+N for the replicating bridge configuration, or 1 encoding and N−1 decodings.
These figures show that the constraints on the bridge and on the terminals are greater on the replicating bridge configuration when the number of participating terminals increases, except with regard to the CPU costs of the mixer bridge. The mixer bridge is penalized on CPU costs, but it should be noted in this respect that the power of this type of bridge currently increases at an even rate.
Another advantage of the mixer bridge over the replicating bridge is that, if the mixer bridge has a large number of coders-decoders (codecs), the audio conference is transparent in codec terms for the terminals since each terminal receives from the bridge only one coded stream suited to its own codec, whereas, in the case of a replicating bridge, the terminals need to have common codecs, and this is all the more difficult to implement when there is a large number N of terminals.
However, the replicating bridge presents the advantage of avoiding a decoding followed by an encoding on the bridge, which guarantees the absence of loss of audio quality and additional delay, unlike the mixer bridge.
Regarding any sound spatialization, the replicating bridge has the advantage of enabling the terminals to perform a customized spatialization since they have individual download streams from the other terminals, whereas the mixer bridge makes it possible to perform the spatialization on the bridge, and not on the terminals, with the advantage of being able to generate a sound scene common to all the participants.
The above comparisons show that, given the respective advantages and drawbacks of the two types of bridge, it is very difficult to implement an audio conference that is totally satisfactory both with respect to audio quality and with respect to the technical constraints notably associated with the capabilities of the terminals to handle a large number of streams.